There is proposed a device (flow cytometer or cell sorter) for obtaining biological information on biological particles by allowing a liquid, which contains dyed biological particles (minute objects to be detected: specimens), to flow into a flow passage of a flow passage-forming member (flow cell), irradiating the biological particles with light illuminated from an illuminating unit (irradiating unit), and detecting scattering light or fluorescent light emitted from the biological particle with a detecting unit (light receiving unit) (for example, see Patent Document 1).
In the past, in connection with the above-mentioned device, a sample flow is adjusted in a manner such that the flow of the liquid flowing through the flow passage of the flow cell is surrounded by a sheath flow and is present near the center of the flow passage, and a light receiving unit is configured such that an optical axis and a focal point of an optical system of the light receiving unit are aligned with and fixed at the center of the flow passage of the flow cell on a plane substantially perpendicular to the sample flow. Further, the irradiating unit is configured such that, when condensed irradation light is irradiated onto the sample flow and scattering light or fluorescent light from the specimens is received, the irradiating unit can optimally adjust the position of the optical axis thereof relative to the sample flow while actually irradiating the sample flow with irradiation light and checking the optical information, in order to obtain scattering light or fluorescent light with high sensitivity or obtain the optical information on specimens with a small variation.